1. Field of the Invention
This invention relates to a light blocking device of such a shape that it can effectively block, out off-axis fluxes passing through a photographing system, both a sagittal peripheral light flux at a given field angle and a tangential peripheral flux at an intermediate field angle, particularly when installed in a portion of the photographing system. The invention is suitable for such photographing systems as used in photo-taking cameras, video cameras and television cameras, and such projection lenses as used in projectors.
2. Related Background Art
The further toward the peripheral area of a field, the more the optical performance of a photographing system or projection lenses generally deteriorates.
Especially in a photographing system having a larger diameter and a wider field angle, the deterioration of the off-axis aberration becomes larger because the optical performance around the peripheral of the field tends to drop significantly.
In order to improve the peripheral optical performance of the field of a photographing system, one or more apertures having a ring-shaped opening, referred to as flare stops herein, are conventionally installed in a portion of the photographing system so that off-axis light fluxes can be partly blocked.
FIG. 3 is an outlined sketch showing a conventional example where a flare stop is installed in a portion within a set of projection lenses for a video projector so that off-axis fluxes can be blocked.
In the photographing field of the figure thereof, the fluxes to be imaged at the point 14 on the axis and at the points 15, 16 and 17 off the axis are the ones within the areas 14a, 15a, 16a and 17a indicated in FIG. 4, respectively. Thus, when a flare stop is installed at the point 3 in FIG. 3, the flux to be blocked by the flare stop will be outside from the area covered by a dotted line 18a in FIG. 4.
In this case, a part of the off-axis fluxes is blocked, but at the same time as indicated in FIG. 5, the on-axis flux (field angle .omega.=0.degree.) as well as an off-axis flux at a narrow field angle (.omega.=.omega..sup.2) are also blocked. This drawback causes a problem that the phototaking system becomes dark as a result of an increase in the F number.
When a flare stop is installed at the point 5 in FIG. 3 where the on-axis flux is not blocked, off-axis fluxes, for example, around the peripheral of the field other than the aimed off-axis fluxes are also blocked, thus inconveniently decreasing the light amount to a great extent.
In Japanese Laid-Open Patent Application Nos. 62-139514.about.139518, a light blocking plate to prevent sagittal flares is disclosed, but this plate has a great difference in light amount between the on-axis and off-axis fluxes.